This invention relates to a speed and torque sensors.
The invention is more particularly concerned with sensors for providing outputs representative of the speed and torque of a shaft by detecting movement of magnetic members mounted on the shaft.
An output representative of the speed of a shaft is readily provided by means of a toothed wheel on the shaft which alters the magnetic field in the region of an electromagnetic sensor, as the teeth on the wheel pass in front of the sensor. The sensor commonly takes the form of a coil wound around a permanent magnet assembly, the coil producing an alternating electrical signal at the same frequency as that at which the teeth pass in front of the magnet assembly. By measuring the frequency or time period, a measure of the shaft speed can be provided.
Torque can be measured by securing two wheels on the shaft at opposite ends of a portion of the shaft of known modulus of elasticity. The wheels are arranged to have teeth that are interdigitated with each other in front of the sensor. An increase in torque causes twisting of that portion of the shaft between the wheels and hence causes angular displacement of one wheel relative to the other. This causes movement of the two sets of teeth relative to one another, altering the separation between them. The torque, therefore produces a shift in the output pulses arising from the two sets of teeth.
It is common practice to have several coils in the sensor, wound together, so that the output from each coil can be used for a different purpose or to provide redundancy. For example, a torque output could be derived from one coil and a speed output derived from another coil. Such sensors can function satisfactorily in normal circumstances with an acceptable accuracy. If, however, one of the coils is shorted, such as by failure in the measuring circuit or interconnecting cables, this causes a relatively high current to be drawn by that coil and thereby alters the magnetic circuit formed by the coils and the permanent magnet assembly. The result of shorting one coil is to alter the voltage/time characteristic of the output of the other coil. Where the other coil provides only a speed signal, this does not result in any adverse effect on accuracy. If, however, the output of the other coil is used to measure torque, this can have an adverse effect on accuracy.